The present invention relates to the field of digital data processing systems, and more particularly, to the field of networks wherein units of the network are connected to a high speed bus requiring arbitration for access and data transfer.
As is well know in the art, as computer networks have grown in size and complexity, the need for high speed data transfer has increased. Among the types of data transfer devices which provide high speed transport between shared resources, such as bridges or switches, is the use of a high speed shared channel or bus. With such an arrangement, network efficiency and utilization is strongly affected by the procedure for controlling access to the high speed bus or channel.
As is also known, in general a transmission of data between two units over a high speed bus or channel requires two steps, since more than one unit has the capability of originating a transmission. The first step is for the transmitting unit to obtain control of the bus for some more or less defined interval. Once the selection step is completed, a second (or transfer) step is used to complete the transfer that is controlled by the selected transmitting unit.
Obtaining control of the bus requires contending with other units desiring bus access, to arbitrate and determine which one will be selected. As is known in the art, there are two principal generic approaches to arbitration. These are central arbitration and distributed arbitration approaches. In the central arbitration, a single, central priority circuit or device receives all the requests for bus access and determines which requesting unit at any given time should be accorded the greatest priority and allowed to use the bus. Once the unit is selected, it is allowed to control the bus and effect the transfer. By contrast, in distributed arbitration, each unit connected to a bus is assigned a specific priority and each unit individually determines whether it has sufficient priority to obtain control of the bus when it desires to do so. If a unit of higher priority simultaneously seeks bus access, a device of lower priority must wait until some later time when it is the highest priority requester.
As is also known, in choosing a method of arbitration, fairness is assured when each unit connected to the bus has substantially equal average priority for obtaining bus access. One such approach to guaranteeing fairness is a primitive round robin method. With such a method, for each unit, two delay interval possibilities exist; the delay selection for each unit is switched from time to time, on a round-robin basis, such that all units are given equal average priority.
In today's networks, data of more than one protocol may also coexist and be transferred from unit to unit in the network via the high speed bus. Examples of different protocols are Ethernet, FDDI, and Asynchronous Transfer Mode (ATM). Therefore, a method of arbitration is required that will not only guarantee fairness, but one that can handle data of both similar and different protocols.